Long-Term Debt - Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Less: current maturities	$ (24,867)	$ (25,346)
Long-term debt, less current maturities	1,998,127	1,798,576
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(24,867)	(25,346)
Long-term debt, less current maturities	1,998,127	1,798,576
Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Long-term debt, less current maturities	1,706,994	1,521,031
Long-term Debt, Current and Noncurrent Abstract
Long-term debt, less current maturities	1,706,994	1,521,031
Notes 3.7% due 2028 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	300,000
IDRBs [Member] \| 2009 Series A, due 2039 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000	50,000
Carrying Amount [Member]
Debt Instrument [Line Items]
Less: current maturities	(24,867)	(25,346)
Long-term debt, less current maturities	1,998,127	1,798,576
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(24,867)	(25,346)
Long-term debt, less current maturities	1,998,127	1,798,576
Carrying Amount [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Long-term debt, less current maturities	1,706,994	1,521,031
Long-term Debt, Current and Noncurrent Abstract
Long-term debt	1,706,994	1,521,031
Long-term debt, less current maturities	1,706,994	1,521,031
Carrying Amount [Member] \| Centuri Construction Group Inc [Member]
Debt Instrument [Line Items]
Less: current maturities	(24,867)	(25,346)
Long-term debt, less current maturities	291,133	277,545
Unamortized debt issuance costs	(1,056)	(1,111)
Long-term Debt, Current and Noncurrent Abstract
Long-term debt	316,000	302,891
Less: current maturities	(24,867)	(25,346)
Long-term debt, less current maturities	291,133	277,545
Centuri secured revolving credit facility	193,301	198,467
Carrying Amount [Member] \| Centuri Secured Revolving Credit Facility [Member] \| Centuri Construction Group Inc [Member]
Long-term Debt, Current and Noncurrent Abstract
Centuri secured revolving credit facility	79,227	56,472
Carrying Amount [Member] \| Centuri Term Loan Facility [Member] \| Centuri Construction Group Inc [Member]
Long-term Debt, Current and Noncurrent Abstract
Centuri secured revolving credit facility	194,357	199,578
Carrying Amount [Member] \| Centuri Other Debt Obligations [Member] \| Centuri Construction Group Inc [Member]
Long-term Debt, Current and Noncurrent Abstract
Centuri other debt obligations	43,472	47,952
Carrying Amount [Member] \| Debentures [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs	(12,480)	(9,350)
Notes payable	1,470,020	1,173,150
Carrying Amount [Member] \| Debentures [Member] \| Notes, 4.45%, due 2020 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	125,000	125,000
Carrying Amount [Member] \| Debentures [Member] \| Notes, 6.1%, due 2041 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	125,000	125,000
Carrying Amount [Member] \| Debentures [Member] \| Notes, 3.875%, due 2022 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	250,000	250,000
Carrying Amount [Member] \| Debentures [Member] \| Notes, 4.875%, due 2043 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	250,000	250,000
Carrying Amount [Member] \| Debentures [Member] \| Notes, 3.8%, due 2046 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	300,000	300,000
Carrying Amount [Member] \| Debentures [Member] \| Notes 3.7% due 2028 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	300,000
Carrying Amount [Member] \| Debentures [Member] \| 8% Series, due 2026 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	75,000	75,000
Carrying Amount [Member] \| Debentures [Member] \| Medium-term notes, 7.78% series, due 2022 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	25,000	25,000
Carrying Amount [Member] \| Debentures [Member] \| Medium-term notes, 7.92% series, due 2027 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	25,000	25,000
Carrying Amount [Member] \| Debentures [Member] \| Medium-term notes, 6.76% series, due 2027 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable	7,500	7,500
Carrying Amount [Member] \| Debentures [Member] \| Revolving Credit Facility and Commercial Paper [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Line of credit facility	39,000	150,000
Carrying Amount [Member] \| IDRBs [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs	(2,026)	(2,119)
Unsecured debt	197,974	197,881
Carrying Amount [Member] \| IDRBs [Member] \| Tax-exempt Series A, due 2028 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Carrying Amount [Member] \| IDRBs [Member] \| 2003 Series A, due 2038 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Carrying Amount [Member] \| IDRBs [Member] \| 2008 Series A, due 2038 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Carrying Amount [Member] \| IDRBs [Member] \| 2009 Series A, due 2039 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Market Value [Member] \| Centuri Construction Group Inc [Member]
Long-term Debt, Current and Noncurrent Abstract
Centuri secured revolving credit facility	195,757	207,588
Centuri other debt obligations	43,256	48,183
Market Value [Member] \| Centuri Secured Revolving Credit Facility [Member] \| Centuri Construction Group Inc [Member]
Long-term Debt, Current and Noncurrent Abstract
Centuri secured revolving credit facility	79,261	56,525
Market Value [Member] \| Debentures [Member] \| Notes, 4.45%, due 2020 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	127,630	129,273
Market Value [Member] \| Debentures [Member] \| Notes, 6.1%, due 2041 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	155,349	158,304
Market Value [Member] \| Debentures [Member] \| Notes, 3.875%, due 2022 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	252,270	256,163
Market Value [Member] \| Debentures [Member] \| Notes, 4.875%, due 2043 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	284,998	283,243
Market Value [Member] \| Debentures [Member] \| Notes, 3.8%, due 2046 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	291,945	302,970
Market Value [Member] \| Debentures [Member] \| Notes 3.7% due 2028 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	302,520
Market Value [Member] \| Debentures [Member] \| 8% Series, due 2026 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	95,542	96,063
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 7.78% series, due 2022 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	28,147	28,714
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 7.92% series, due 2027 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	30,810	31,542
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 6.76% series, due 2027 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	8,811	8,882
Market Value [Member] \| Debentures [Member] \| Revolving Credit Facility and Commercial Paper [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	39,000	150,000
Market Value [Member] \| IDRBs [Member] \| Tax-exempt Series A, due 2028 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000	50,000
Market Value [Member] \| IDRBs [Member] \| 2003 Series A, due 2038 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000	50,000
Market Value [Member] \| IDRBs [Member] \| 2008 Series A, due 2038 [Member] \| Southwest Gas Corporation [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	$ 50,000	$ 50,000

Debt instrument unamortized discount and debt issuance costs.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Fair value portion of contractual arrangement with a lender under which borrowings can be made up to a specific amount at any point in time.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Including the current and noncurrent portions, aggregate carrying amount of all types of notes payable, as of the balance sheet date, with initial maturities beyond one year or beyond the normal operating cycle, if longer.

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Fair value portion of notes payable.

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Amount of long-term debt classified as other.

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Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of uncollateralized debt obligations (with maturities initially due after one year or beyond the operating cycle if longer).

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